1. Technical Field
This invention relates to vapor deposition apparatus in general, and to cathodic arc vapor deposition apparatus in particular.
2. Background Information
Vapor deposition as a means for applying a coating to a substrate is a known art that includes processes such as chemical vapor deposition, physical vapor deposition, and cathodic arc vapor deposition. Chemical vapor deposition involves introducing reactive gaseous elements into a deposition chamber containing one or more substrates to be coated. Physical vapor deposition involves providing a source material and a substrate to be coated in a evacuated deposition chamber. The source material is converted into vapor by an energy input, such as heating by resistive, inductive, or electron beam means.
Cathodic arc vapor deposition involves a source material and a substrate to be coated placed in an evacuated deposition chamber. The chamber contains only a relatively small amount of gas. The negative lead of a direct current (DC) power supply is attached to the source material (hereinafter referred to as the "cathode") and the positive lead is attached to an anodic member. In many cases, the positive lead is attached to the deposition chamber, thereby making the chamber the anode. A arc-initiating trigger, at or near the same potential as the anode, contacts the cathode and subsequently moves away from the cathode. When the trigger is still in close proximity to the cathode, the difference in potential between the trigger and the cathode causes an arc of electricity to extend therebetween. As the trigger moves further away, the arc jumps between the cathode and the anodic chamber. The exact point, or points, where an arc touches the surface of the cathode is referred to as a cathode spot. Absent a steering mechanism, a cathode spot will move randomly about the surface of the cathode.
The energy deposited by the arc at a cathode spot is intense; on the order of 10.sup.5 to 10.sup.7 amperes per square centimeter with a duration of a few to several microseconds. The intensity of the energy raises the local temperature of the cathode spot to approximately equal that of the boiling point of the cathode material (at the evacuated chamber pressure). As a result, cathode material at the cathode spot vaporizes into a plasma containing atoms, molecules, ions, electrons, and particles. Positively charged ions liberated from the cathode are attracted toward any object within the deposition chamber having a negative electric potential relative to the positively charged ion. Some deposition processes maintain the substrate to be coated at the same electric potential as the anode. Other processes use a biasing source to lower the potential of the substrate and thereby make the substrate relatively more attractive to the positively charged ions. In either case, the substrate becomes coated with the vaporized material liberated from the cathode. The rate of deposition, the coating density, and thickness can be adjusted to satisfy the needs of the application.
The random movement of the arc can sometimes lead to non-uniform erosion of the cathode, which in turn can limit the useful life of the cathode. To avoid non-uniform erosion, it is known to steer or drive the arc relative to the cathode. U.S. Pat. Nos. 4,673,477, 4,849,088, and 5,037,522 are examples of patents that disclose apparatus for steering an arc relative to a cathode. Some prior art steers the arc by mechanically manipulating a magnetic field source relative to the cathode. Other prior art steers the arc by electrically manipulating the magnetic field source relative to the cathode. In both these approaches, the speed of the arc relative to the cathode is limited by the speed of the apparatus mechanically or electrically manipulating the magnetic field source. Another limitation of these approaches is the complexity of the hardware or switching mechanisms necessary to manipulate the magnetic field source relative to the cathode. A person of skill in the art will recognize that a production coating environment is harsh and simplicity generally equates with reliability.
Many prior art cathodic arc coaters are not designed for efficient, cost effective high volume use. In the example above, where complex switching mechanisms and hardware are used to manipulate magnetic field sources, cathode replacement can be cumbersome and time consuming, and consequently costly. The same cost-problem exists in most cathodic arc coaters that directly cool the cathode. Direct cooling is generally accomplished by passing coolant between the cathode and a manifold attached to the cathode, or by piping coolant directly through the cathode. Either way, the cathode must be machined to accept the manifold or piping, and the cost of the consumable cathode is consequently increased (often dramatically). In addition, some desirable cathode materials are not readily machinable. Another problem with directly cooling the cathode is the labor required to replace the cathode when its useful life has expired. In the previous example where a manifold (or piping) is mechanically attached to the cathode, the hardware (or piping) must be detached from the old cathode and attached to a new one, and the deposition chamber subsequently cleaned of coolant. For those applications which require cathode replacement after each coating run, the cost of the cathode and the labor to replace it can be considerable.
In short, what is needed is an apparatus for cathodic arc vapor deposition of material on a substrate that operates efficiently, one capable of consistently providing a high quality coatings on a substrate, and one that operates cost effectively.